This invention relates to high intensity mercury halide lamps, and particularly to improving such lamps so that they can have longer lamp life and be more useful as a lamp in a lightweight mobile lighting fixture that does not need heavy, wind load increasing cooling systems.
High pressure mercury vapor discharge lamps with and without additional chemical additives are well known in the prior art, see for example U.S. Pat. No. 3,654,506. In the typical high intensity mercury halide lamp, a vitreous quartz lamp envelope is filled with argon gas, mercury, plus other metal salts. Protruding into the lamp envelope are two tungsten electrodes, each electrode being connected to a molybdenum foil which is in turn connected to a follower rod which serves as an electrical termination.
High wattage mercury halide lamps were developed in Germany by OSRAM GmbH and are sold by them under the trademark HMI lamps. The HMI lamp is known to be of rugged construction. The quartz envelope has great inherent strength, and there are no especially fragile internal parts. Under normal operating conditions neither shock nor vibration have any significant deleterious effect on the lamp. Because HMI lamps are lightweight and produce high intensity light with a small amount of lamp weight, they are useful for lighting needs in the sports industry and stage and studio uses, where lights are often mounted on a lighting rack in an elevated position. When operated in sealed weatherproof fixtures, temperatures high enough to cause seal oxidation and subsequent failure require the use of external cooling of the lamp ends.
When lights are used in sports lighting and mounted on an elevated rack, the weight of the lamp reflector is a special concern. This is important because such lights, especially in the field of sports lighting are often subjected to high winds. Obviously, the heavier the load the more susceptible to wind damage.
There has therefore been a continuing need for the development of lightweight luminaire assemblies which can be usefully employed in sports lighting, and especially useful for mobile sports lighting, which requires compactness, intense light source, ruggedness and ease of operation. The assignee of the present application has heretofore combined the use of HMI lamps and luminaire assemblies for the special needs of mobile lighting, see for example U.S. Pat. No. 4,423,471 of Dec. 27, 1983, issued to Myron K. Gordin et al.
The Gordin patent, while presenting a successful lighting fixture and luminaire assembly using lightweight high intensity HMI lamps, solves the problem of high temperatures in the HMI lamp by use of an air cooling assembly. In the application described in the Gordin et al. patent, the air cooling assembly was needed in order to provide longer lamp life for HMI lamps when used in the luminaire assemblies there described.
It necessarily follows that providing a fan and cooling assembly itself is costly, increases the weight of the unit, and thus while it increases lamp life, also brings its own accompanying problems.
It is a primary objective of the present invention to provide an improved high wattage mercury halide lamp which can be used in the manner described in U.S. Pat. No. 4,423,471 of Dec. 27, 1983, without the need for an independent cooling system.
Another objective of the present invention is to provide a lamp assembly for high wattage mercury halide lamps which slows seal degradation caused by lead oxidation, as explained below.
High intensity mercury halide lamps of the general construction described herein have electrical termination rods which protrude from the lamp side arm. These are connected to electrical leads and caps placed thereon. These high intensity mercury halide lamps are filled with mercury and argon to achieve an arc discharge. Typically they also have metal salts added to the arc tube to provide good spectral balance and color rendition similar to sunlight within the tube. These are well-known. The HMI lamp envelope is unique in that it has a very small internal dimension and very thick walls in order to achieve the high temperatures needed to allow an active halogen cycle to remove evaporated tungsten from the inner quartz wall and redeposit it onto the tungsten electrodes.
It has been discovered that where the temperature at which the outboard electrical rod or pin joins the quartz exceeds 400.degree. C. in the presence of air, the molybdenum ribbon of the electrical ribbon assembly will begin to oxidize. When this occurs, since the oxide has larger molecules than the molybdenum itself, it causes an increase in pressure. This increase in pressure gradually will lever the seal of the ribbon assembly open, causing cracks. Eventually this causes lamp fatigue and failure. However, if the lamp assembly is cooled, as for example by the cooling system described in U.S. Pat. No. 4,423,471, preferably to keep the temperature below 400.degree. C., but ideally below 250.degree. C., the failures will be significantly minimized and lamp life extended.
Thus, an important advantage and objective of the present invention is to provide a ductile brazing alloy, which it has been discovered will effectively seal the electrode follower rod to the vitreous quartz lamp arm by a hermetic seal which will not be destroyed by heat, and which a ductility sufficient to compensate for the expansion mismatch between the vitreous quartz and the refractory metal follower rod over the extremes of temperature experienced by the lamp during operation. The result is longer lamp integrity and thus longer life. The achievement of this improved high wattage mercury halide lamp, coupled with the construction and arrangement which allows the use of applicant's discovered certain brazing alloys in combination with the high intensity mercury halide lamps for use in sports lighting without a cooling system is another primary objective of the present invention.